The present invention relates to an apparatus and methods for multiplexing two video signals and, in particular, to an apparatus and method which multiplexes an external source of program material with the program material normally received by a television antenna for the purpose of simultaneously providing these two sources of program material to the antenna input terminal of the television set.
It has recently become very popular to pay private services a fee to provide television program material not otherwise available for viewing on the channels normally received by the television antenna. While certain of these services offer a plurality of channels from which to select a desired television program for viewing, many offer only a single channel of program material, such as subscription television offered under the trademark "ON-TV".
Program material for "ON-TV" is transmitted during portions of the day over UHF channel 52. The signal for this material, however, is transmitted in a scrambled format and therefore cannot be viewed without an unscrambling device known as a "decoder box".
In using the television set, of course, the viewer sets the channel selector to one of the many channels which are available for viewing. The ability to select channels is made possible by the fact that each channel of program material occupies a certain exclusive segment of the radio frequency spectrum band. Thus, changing the channel selector merely alters the segment of the radio frequency spectrum band which the television set is tuned to view. Of course, should a channel be selected which, in fact, is not being used by any local transmitting facility, no intelligible picture or sound will emanate from the television set. Rather, "snow" will appear on the television picture screen accompanied by a hissing sound. A channel which is not utilized by any transmitting facility within the receiving area is hereinafter referred to as an "unused channel".
To enable the "ON-TV" "decoder box" to unscramble the television signal, it must be inserted in the cable which connects the terminal of the television set to the television antenna. Typically, this is accomplished by disconnecting the antenna wire from the television antenna terminal on the television set and connecting it instead to the input of the "decoder box". The output of the "decoder box" is then connected to the antenna terminal of the television set by a separate piece of antenna wire.
In operation, the "decoder box" unscrambles the scrambled television signal being transmitted over UHF channel 52 and then retransmits it on VHF channel 3. Accordingly, to view the unscrambled "ON-TV" signal, the channel selector of the television set must be set to channel 3.
Because of the way the "decoder box" operates, it is not possible to view any other television channel, except "ON-TV" channel 3, while the "decoder box" is switched on. Conversely, it will only be possible to view the channels normally received by the television antenna when the "ON-TV" "decoder box" is switched off. The "ON-TV" channel will not be available. Accordingly, the television's normally used channel selector cannot switch between all available channels of viewing material on its own. Rather, it will also be necessary to switch the "ON-TV" "decoder box" on or off to effectuate viewing of all available television channels.
In addition to the inconvenience of having to operate a second switch to enable viewing of all available television channels, the commonly used remote-control channel selector can no longer be fully used for its intended purpose. This is because the remote-control channel selector of the television set is not generally capable of additionally switching the "ON-TV" "decoder box". Thus, when it is desired to switch from viewing channels received by the normally-used television antenna to the "ON-TV" channel, i.e., channel 3, not only must the channel selector be changed by the remote-control channel selector to channel 3, but it will be additionally necessary to manually switch the "ON-TV" "decoder box" on.
It will also not be possible with an "ON-TV" "decoder box" to utilize a standard video tape recorder to record a channel received by the outdoor television antenna while the "ON-TV" signal is being viewed on the television set, or vice-versa. This is because the output from the "ON-TV" "decoder box" does not simultaneously include both the "ON-TV" programming material and the remaining channels received by the normally used television antenna.